In recent years, along with development of the digital technology, electronic devices such as portable information devices and information home appliances have increasingly higher-level functionalities. There is thus a higher demand on variable resistance elements for an increase in capacity, a reduction in power for writing, an increase in speed for writing/reading, and a longer operating life.
In response to such a demand, it is said that there is a limit on the miniaturization of existing flash memories using floating gates. In contrast, a variable resistance element using a variable resistance layer as a material of a memory unit (i.e., a variable resistance memory) can be composed of a simple-structured memory element represented by a two-terminal variable resistance element (i.e., a nonvolatile memory element), which therefore lays high expectations for further miniaturization, increase in speed, and reduction in power consumption. The variable resistance layer which is used as a material of the memory unit will have resistance changing in value from high resistance to low resistance or from low resistance to high resistance by input of electric pulses or the like, for example. In this case, to obtain stable memory characteristics, it is necessary that two values of low resistance and high resistance be clearly distinguished, a change between low resistance and high resistance be stable at high speed, and these two values be held in a nonvolatile manner.
Thus, various proposals have been made on the nonvolatile memory element in order to realize stable memory characteristics and miniaturization of a memory element. As an example of a variable resistance element included in such a nonvolatile memory element, there is proposed a variable resistance element in which transition metal oxides having different oxygen content atomic percentages are stacked and used as a variable resistance layer. For example, Patent Literature (PTL) 1 discloses a variable resistance element which selectively causing the occurrence of oxidation/reduction reaction in an electrode interface which is in contact with a variable resistance layer having a high oxygen content atomic percentage, to stabilize resistance change. The variable resistance element includes a lower electrode, a variable resistance layer, and an upper electrode, and a memory array is configured from a two-dimensional or three-dimensional array of such variable resistance element. Furthermore, in the variable resistance element, the variable resistance layer is of a stacked structure including a first variable resistance layer and a second variable resistance layer, and the first and second variable resistance layers comprise the same type of transitional metal oxide. Furthermore, the oxygen content atomic percentage of the transition metal oxide comprised in the second variable resistance layer is higher than the oxygen content atomic percentage of the transition metal oxide comprised in the first variable resistance layer. By adopting such a structure, when voltage is applied to the variable resistance element, most of the voltage is applied to the second variable resistance layer which has a higher oxygen content atomic percentage and exhibits a higher resistance value. Furthermore, oxygen, which can contribute to the reaction, is abundant in the vicinity of the interface. Therefore, oxidation/reduction reaction occurs selectively at the interface between the upper electrode and the second variable resistance layer, and stable resistance change can be realized.